Known from DE GMS 202 09 525.8 is a coaxial cannula that can be inserted into tissue and in which a biopsy needle unit can be employed. A seal is provided between the interior wall of the coaxial cannula and the exterior wall of the biopsy needle unit in order to, first, prevent fluid from escaping and, second, to make it possible to create a vacuum in the tissue to be biopsied. DE GMS 202 09 525.8 states that the sealing function of the seal must be created such that it prevents air from entering or escaping and also prevents fluid from escaping.
Such a seal has led to problems in practice. When inserting the biopsy needle unit into the coaxial cannula and during subsequent positioning of the needle unit, the air that has penetrated into the coaxial cannula as a result of the insertion process is sealed in and air bubbles form that cause problems with the ultrasound or MR images made while the needle is being positioned so that precise positioning is not possible due to the air occlusions.